Dwarf plants have had a major impact on agriculture. Dwarf varieties of wheat are widely used in North America due to both reduced potential for lodging and high yields. There are other benefits that may be realized from the use of dwarf crop plants including reductions in the amounts of pesticides and fertilizers required, higher planting densities, and reduced labor costs.
In view of the current trends of both increasing human population and the decreasing land area suitable for agriculture, increasing agricultural productivity is, and will continue to be, a challenge of paramount importance. Dwarf crop plants have been and will continue to be important components of our agricultural production system. Increased usage of dwarf crop plants may help to meet the agricultural production demands of the future. However, commercially acceptable dwarf varieties are not available for all crops.
In addition to the use of dwarf plants to control plant height, synthetic chemicals are routinely applied to certain economically important plant species to reduce growth. Plant growth regulators known as growth retardants are used to reduce stem elongation in a variety of crops including cotton, grape vines, fruit trees, peanuts, wheat and ornamentals such as azaleas, chrysanthemums, hydrangeas, poinsettias and many bedding plants. All of the commonly used growth retardants are inhibitors of gibberellin biosynthesis and limit stem or shoot growth by reducing elongation. In the United States, the most widely used growth retardant is mepiquat chloride, which is registered for use on cotton. Benefits attributed to the use of mepiquat chloride on cotton include increased yield, improved defoliation, improved stress tolerance, more uniform crop maturity and the ability to harvest earlier. Previously, the growth retardant daminozide was registered for use in the United States on apples, grapes and peanuts under the trademarks ALAR and KYLAR but was removed from use on food crops due to human health concerns. Despite the demands of agricultural producers for a product to replace diaminozide, there are no growth retardants registered for use on grapes, fruit trees and peanuts in the United States. Daminozide, however, is still widely used on certain non-food, plant species.
Uncovering the molecular mechanisms that control plant growth processes such as cell division and cell elongation will likely aid in the development of new plant varieties with reduced stature and new methods for reducing plant growth. Such new plant varieties and methods may provide both farmers and horticulturists with environmentally benign alternatives to the use of synthetic growth-retarding chemicals.
Elongation of plant cells and organs is one of the most critical parameters of plant growth and development. Regulation of this trait in plants, however, is a fairly complicated process, as both external and internal factors influence it. The most important external stimulus is light, with its normally repressible or negative effect on cell elongation (Quail, P. H. (1995) Science 268:675-680; Kende, et al., (1997) Plant Cell 9:1197-1210). The internal control of cell elongation is mediated by a number of chemicals, normally referred to as plant growth regulators or hormones (Kende, et al., (1997) Plant Cell 9:1197-1210). Among the classical plant hormones, auxins and gibberellins (GAs) both promote cell elongation whereas cytokinins and abscisic acid each have been shown to have a negative effect on cell elongation (Kende, et al., (1997) Plant Cell 9:1197-1210). Recently, another class of plant growth regulators, named brassinosteroids, has been identified that also dramatically promote plant growth (Yokota, T. (1997) Trends Plant Sci. 2:137-143; Azpiroz, et al., (1998) Plant Cell 10:219-230; Choe, et al., (1998) Plant Cell 10:231-243). However, the mechanisms by which plant hormones act, either singly or in concert, to control cell elongation remains unclear.
One way to gain an understanding of mechanisms that mediate cell elongation is to study mutants in which this aspect of plant growth is compromised (Klee, et al., (1991) Annu. Rev. Plant Physiol. Plant Mol. Biol. 42:529-551). Numerous such mutants have been identified across most plant species, including maize, in which more than 25 single-gene mutations that affect plant stature have been characterized (Coe, et al., (1988) In: Corn & Corn Improvement, G. F. Sprague (Ed.) Madison, Wis.; Sheridan, W. F. (1988) Annu. Rev. Genet. 22:353-385). These dwarf mutants are considered to be GA related, mainly because GA is the only phytohormone whose role in regulating height in maize has been convincingly established (Phinney, et al., (1985) Curr. Top. Plant Biochem. Physiol. 4:67-74; Fujioka, et al., (1988) Proc. Natl. Acad. Sci. USA 85:9031-9035). Both types of mutants, GA responsive and GA non-responsive, have been found in this collection of maize mutants. While genes for a number of GA-responsive mutants have been cloned and found to be involved in GA biosynthesis (Bensen, et al., (1995) Plant Cell 7:75-84; Winkler, et al., (1995) Plant Cell 7:1307-1317), less is known about the nature of defects in GA non-responsive maize mutants.
DELLA proteins are keystones of the gibberellin (GA) signal transduction cascade, acting as negative regulators of the GA response that are degraded in the presence of elevated GA concentrations (Silverstone, et al., (2001) Plant Cell 10:155-169). DELLA domain proteins are of particular interest because of the gibberellin insensitive dwarf phenotype of their gain-of-function mutants, which were partially responsible for the “Green Revolution” by way of their increase in wheat harvest index (Peng, et al., (1999) Nature 400:256-261). Mutations in the N-terminal DELLA domain often cause a dominant GA-insensitive phenotype by greatly increasing the stability of this negative regulator of GA signal transduction (Silverstone, et al., (2001) Plant Cell 10:155-169; Gubler, et al., (2002) Plant Physiol. 129:191-200; Itoh, et al., (2002) Plant Cell 14:57-70). Recently, Griffiths, et al., ((2006) Plant Cell 18:3399-3414) demonstrated that both N-terminal regions I and II are required for DELLA protein interaction with Arabidopsis GID1a. C-terminal mutations in the conserved GRAS domain typically lead to loss-of-function (Dill, et al., (2004) Plant Cell 16:1392-1405), constitutive GA growth response phenotype with the notable exception of a recently identified Brassica rapa mutant Brrga1-d (Muangprom, et al., (2005) Plant Physiol. 137:931-938) and the barley sln1c mutant (Gubler, et al., (2002) Plant Physiol. 129:191-200).
To keep up with the demand for increased agricultural production, new targets are needed for genetically engineering agricultural plants for the improvement of agronomic characteristics. The isolation and molecular characterization of genes encoding proteins that are involved in controlling cell division and elongation in plants will provide new targets for agricultural scientists to manipulate.